Milestone: Carbon-14 discovered
Date: 27 February 1940
Where: Berkeley, California
WHO: Martin Kamen and Samuel Ruben
On this day in 1940, two scientists discovered an elusive form of carbon – inadvertently opening a window into lost civilizations.
But Ernest Lawrence, who founded Berkeley Laboratory, was determined to find it. In 1939, he tasked chemists Martin Kamen and Samuel Rubin with discovering carbon-14. For a year they found no hint of the elusive atom.
Then, in January 1940, they began a “desperation experiment,” placing a piece of graphite (a crystalline form of carbon) inside a cyclotron, one of the first types of particle accelerators. The cyclotron bombarded their sample with deuterons – nuclei of a heavy form of hydrogen with one proton and two neutrons. The hope was that the crystalline form of carbon would absorb the extra neutrons, emit a proton and become a “heavy” version of carbon.
They ran the experiment for 120 hours straight. On February 15, a sleep-deprived Kamen stopped bombarding the sample with deuterons and went home. He was so troubled that the police, who were looking for an escaped murderer, briefly questioned him.
When Kamen was released, he returned to the laboratory, where his colleague Ruben noted faint signs of radioactivity in the sample. Over the next two weeks, they purified the carbon, converting it to a CO2 gas that could be pumped at the right angle by the Geiger counter to measure the radioactivity.
Surprisingly, the carbon did not have a short half-life – the time it takes for half of the radioactive atoms to decay into a stable atom.
“The measured cross section combined with the low yield suggests that the half-life is very long (years),” the researchers wrote in a short paper published March 15, 1940 in the journal Physical review letters.
Their measurements indicated that it would take about 4,000 years for about half of the carbon-14 to decay to nitrogen-14. (We now know that the half-life of carbon-14 is about 5730 years.)
Even at the time, they recognized the significance of the discovery.
“Long-lived radiocarbon will be of great importance for many chemical, biological and industrial experiments,” the researchers wrote in the paper.
Over the next few years, Ruben and Kamen used radioactive carbon and oxygen molecules to illuminate the key steps in photosynthesis. Unfortunately, in 1943, Ruben died in a laboratory accident while working with a poisonous gas, and Kamen was fired from Berkeley after having social interactions with musicians and other people considered “leftist” during the Red Scare. In 1948 he was hauled up to testify before the House Un-American Activities Committeeand although he was never found guilty of any wrongdoing, he was dogged by unfounded allegations for years.
Although the implications of Kamen and Ruben’s experiments were immediately apparent, they were not until 1949 that University of Chicago chemists James Arnold and Willard Libby demonstrated that the ratio of carbon-14 to stable carbon could be used to estimate the age of carbonaceous relics. Libby would serve 1960 The Nobel Prize in Chemistry for his work on radiocarbon dating.
Archaeologists routinely use radiocarbon dating to estimate the age of ancient skeletons and other objects up to 50,000 years old. And newer techniques analyze radioactive isotopes of elements such as strontium and lead to determining where ancient people lived and died, what they ate and what pollutants they had encountered during their lives.
